C ++：向量问题向量

Question

I wrote a c++ code where i'm testing the running time of vector push_back. I have a vector of vector. I called my main vector, mainVec, and embedded vector, subVec. So, I push backed 2^20 elements into subVec and then push backed subVec 2^20 times into mainVec. However, in the loop of subVec-push_back I have a cout command which doesn't get executed. I was hoping you can point out my mistake.

Here is the code (There is no error in the code, though):

vector<int> subVec; 
vector< vector<int> > mainVec;

//Fills the subvector with 2^20 elements
for( size_t i = 0; i < (pow(2,20)+1); ++i) subVec.push_back(i);

//Filling of the maiVec with 2^20 subVec
for( size_t j = 10; j < 21; ++j) {
    cout << pow(2,j) << endl;
    clock_t t1 = clock();

    //2^j times subVec is push_backed for j < 21
    for( size_t k = 0; k < pow(2,j); ++k ) mainVec.push_back( subVec );

    t1 = clock()-t1;

    //Outputting to file
    cout << "\t" << (float(t1) / CLOCKS_PER_SEC) << endl;
    //ofs << pow(2,j) << "\t\t" << (float(t1) / CLOCKS_PER_SEC) << endl;
}

Answer 1

There are several issues with your code.

First, you don't need the +1 in the first loop, ie,. pow(2,20)+1 . Since you're starting with 0 and you want 2^20 times, you need to do until i<2^20 .

Second, it's better to calculate the pow s before the loop, otherwise it will calculate them each time and that could take forever.

Third, you can do 1<<j instead of pow(2,j) . Just FYI.

Forth, and most important, we are talking about a tremenduous amount of memory here. Even your smallest loop is doing 2^30 ints which is 4GB of memory. My guess is that your program is just killing your computer and the reason it never prints the second cout is that it doesn't get there (because it's trying to use swap file for the memory). Try using smaller numbers, say 2^10 for the first loop, and see if you get the outputs.